To efficiently use water resources, there have been developed techniques of membrane-filtering various wastewaters to give reusable water. For example, Patent Documents 1 and 2 disclose methods for membrane-filtering wastewater used for washing a pool of a water purifying plant to reuse it as washing water. The methods described in Patent Documents 1 and 2 are intended to treat wastewater having relatively good quality. When wastewater is simply subjected to membrane-filtering, depending on the quality of the wastewater, these methods may become unable to work due to clogging of a membrane with organic matters and the like in the wastewater in a short time.
Techniques for preventing such clogging of a membrane include a method for adding ozone to raw water to decompose organic matters that cause clogging of a membrane, as described in Patent Documents 3 and 4. However, sufficient decomposition of organic matters requires a large amount of ozone. In Patent Document 3, in order to make ozone remaining on the surface of the membrane in an amount sufficient to decompose organic matters, a large amount of ozone is supplied such that a concentration of dissolved ozone in filtered water having passed through the membrane is 0.5 mg/L. In Patent Document 4, although a concentration of dissolved ozone in filtered water having passed through the membrane is 0.05 to 1.0 mg/L, ozone is further added to the filtered water in a subsequent process. Ozone generation is electricity guzzling, and thus addition of large amount ozone has a problem of high running cost. In addition, decomposition of organic matters progresses with a large amount of ozone to allow organic matters to pass through the membrane. These methods thus have a problem that when raw water is wastewater containing organic matters, filtered water having passed through the membrane has high concentration of remaining organic matters. For these reasons, there is little use of ozone to give reusable water from wastewater by membrane-filtration. Patent Documents 3 and 4 are intended to purify relatively clean raw water.
Patent Document 1: JP-A-1999-235587
Patent Document 2: JP-A-2001-87764
Patent Document 3: JP-A-2003-285059
Patent Document 4: Japanese Patent No. 3449248